The objective was to examine the longitudinal correlates of sluggish cognitive tempo (SCT) and attention deficit/hyperactivity disorder (ADHD)-Inattention (IN) dimensions with mothers' and fathers' ratings of Spanish children. Mothers and fathers rated SCT, ADHD-IN, ADHD-hyperactivity/impulsivity (HI), oppositional defiant disorder (ODD), depression, academic impairment, and social impairment on 3 occasions (twice in first-grade year [6-week separation] and once in the second-grade year [12 months after the first assessment]) in Spanish children (758, 746, and 718 children at the 3 time-points with approximately 55% boys). The results showed that (a) higher levels of SCT from earlier assessments predicted higher levels of depression, academic impairment, and social impairment at Assessment 3 after controlling for ADHD-IN at earlier assessments; (b) higher levels of ADHD-IN from earlier assessments predicted higher levels of depression, academic impairment, and social impairment at Assessment 3 after controlling for SCT at earlier assessments; (c) higher levels of ADHD-IN from earlier assessments predicted higher levels of ADHD-HI and ODD at Assessment 3 after controlling for SCT from earlier assessments; and (d) higher levels of SCT from earlier assessments either showed no unique relationship with ADHD-HI and ODD or predicted lower levels of ADHD-HI and ODD at Assessment 3 after controlling for ADHD-IN from earlier assessments. Initial evidence is provided of SCT's unique longitudinal relationships with depression and academic/social impairment and different longitudinal relationships with ADHD-HI and ODD relative to ADHD-IN, thus adding to a growing body of research underscoring the importance of SCT as distinct from ADHD-IN.